To produce casting molds and cores, frequently binder systems on the basis of polyurethane are used. These are two-component systems, one component of which consists of polyols with a minimum of two OH groups in the molecule and the other of polyisocyanates with a minimum of two NCO groups in the molecule. These two components, in dissolved form, are added to a basic granular molding material (in most cases sand) and are subjected to a curing reaction by adding a catalyst.
In a typical example of such systems, the polyol is a precondensate of phenol or phenol compounds with aldehydes which contains free OH groups (hereinafter referred to as xe2x80x9cphenolic resinxe2x80x9d), and the polyisocyanate is an aromatic polyisocyanate, such as diphenylmethanediisocyanate. Tertiary amines are used as catalysts. Depending on whether the cold-box process or the nobake process is used, the catalyst, in combination with the remaining ingredients of the binder system, is added either immediately prior to processing the molding material mixture or after the molding material mixture, which is initially produced without catalyst, has been added into a mold in which the mixture is gassed with gaseous amine.
In this type of system, solvents are required to ensure that during mixing with the basic molding material, the components of the binding agent are maintained at a sufficiently low viscosity. This is particularly true with respect to phenolic resins which, due to their higher viscosity always require a solvent, but it also applies to polyisocyanates. One problem encountered in this context is that the two binder components require different types of solvents. Thus, as a rule, nonpolar solvents work well with polyisocyanates but are not readily compatible with phenolic resins, and the reverse applies to polar solvents. In practice, it is therefore common to use mixtures of polar and nonpolar solvents which are balanced specifically for the binder system used. In this context, it should be ensured that the boiling range of the individual components of this mixture is not too low so that the solvent does not turn prematurely ineffective due to evaporation.
The nonpolar solvents preferably used so far were high-boiling aromatic hydrocarbons (mainly in the form of mixtures) with a boiling range above approximately 150xc2x0 C. at normal pressure, and the polar solvents used were, among other things, certain sufficiently high-boiling esters, such as the xe2x80x9csymmetricalxe2x80x9d esters described in the German Patent Specification No. 2,759,262, the acid residue and the alcohol residue of which contain a relatively large number of C atoms within the same range (approximately 6-13 atoms).
In spite of all the advantages of polyurethane binders for foundry technology, these binders have one serious drawback In that they are responsible for evaporations and the gas evolution in the working place, which, in most cases, cannot be prevented by protective measures, such as fume hoods, or similar devices. As a result of the fact that, in the meantime, it was possible to reduce the residual content of free formaldehyde and free phenol, the development in the area of resins has led to products which cause very low workplace exposure; and even with respect to the esters which, by nature, have a disagreeable smell, it has been possible to improve the situation markedly by the use of the symmetrical esters mentioned above, but what remains is the problem of exposure to the high-boiling aromatic hydrocarbons in the working place, which so far could not be dispensed with. These aromatic hydrocarbons are generally alkyl-substituted benzenes, toluenes, and xylenes. To ensure the highest possible boiling point, however, they may, in addition, also contain compounds with condensed benzene rings, i.e., naphthalene, etc., which are substances considered hazardous to human health and which are released not only after casting but already during the production of the molding material mixtures.
This problem is to be solved by this invention. Briefly, this is achieved according to this invention through the use of methyl esters of higher fatty acids as the solvent or solvent component for the individual or both components of the polyurethane binders. In this context, the term xe2x80x9cmethyl esters of higher fatty acidsxe2x80x9d, hereinafter referred to as xe2x80x9cfatty acid methyl estersxe2x80x9d, includes all monomethyl esters of fatty acids having a carbon chain of 12 C atoms or more. These methyl esters can be readily prepared by transesterification of fats and oil of vegetable of animal origin which are normally available in the form of triglycerides or can be prepared without problems by esterification of fatty acids obtained from such fats and oils.
Rapeseed oil methyl ester is a typical example of an ester on the basis of vegetable oils; it is a suitable solvent, particularly since it is available at low cost in the form of diesel fuel. But the methyl esters of other vegetable oils, such as soybean oil, linseed oil, sunflower oil, peanut oil, tung oil, palm kernel oil, coconut oil, castor oil and/or olive oil, can also be used. In addition, marine animal oil, tallows, and animal fats can also serve as starting materials for methyl esters that are to be used according to this invention.
The fats and oils which serve as starting materials can be used in random mixtures. They need not be either fresh and pure natural products, but may be used in the form of hydrogenated fats and oils or those which have been otherwise modified in the C chain. Even waste oils and waste fats, e.g., used table oils or oils used for frying, can be used as starting materials for the methyl esters that are to be used according to this invention. Thus, a further aspect of this invention is to make use of waste materials that are harmful to the environment.
The invention is based on the surprising discovery that the fatty acid methyl esters which are polar solvents can surprisingly perform, in a very outstanding manner, the function of the nonpolar solvents required to date and can thus entirely or substantially replace these. Thus, it is possible for the first time to offer a solvent which can be suitably used for both components of a polyurethane binder system and which, at the same time, may make the use of nonpolar solvents, especially of high-boiling aromatic hydrocarbons completely superfluous. In view of the fact that it was so far not possible to use any of the polar solvents proposed for use in polyurethane binder systems without the addition of nonpolar solvents, this finding was not to be expected.
A 100% replacement of the high-boiling aromatics by fatty acid methyl esters is to be preferred especially for environmental protection reasons since in this case, the ecological advantages of this invention can be fully utilized. It is, however, also possible to use these methyl esters together with high-boiling hydrocarbons If this should be expedient in individual cases. If the amount of the fatty acid methyl esters exceeds the amount of the hydrocarbons, the ecological advantages of the invention are still sufficiently evident, although to a degree which gradually decreases. Overall, the Invention thus provides an environmentally compatible variant of the conventional binder/solvent systems, even when the methyl esters are used together with relatively small amounts of aromatics, said variant not being inferior to these conventional systems. It is of course also possible to use solvents containing fatty acid methyl esters and high-boiling aromatics, in which, conversely, the amount of aromatics predominates over the amount of fatty acid methyl esters, but in this case the ecological advantages of the invention are no longer sufficiently evident.
In addition, in certain cases It may be useful to also add an additive, which increases the polarity of the solvent, to the solution of the phenolic resin in the methyl ester. Suitable for this purpose are many polar components, for example a mixture of dimethyl esters of dicarboxylic acids with 4 to 6 carbon atoms, also known as xe2x80x9cdibasic estersxe2x80x9d, abbreviated as xe2x80x9cDBExe2x80x9d. The use of this type of polarizing additive in no way entails a change of the basic advantages obtained when fatty acid methyl esters are used as solvents for polyurethane binder systems.
The rapeseed oil methyl ester mentioned above as a typical example of the solvents to be used according to this invention is an environmentally harmless and natural CO2-neutral product. It is high-boiling and sufficiently thin-bodied, i.e., it meets the physical requirements of a solvent for polyurethane binder systems. In addition, it is also nearly odor-free and considered to be harmless with respect to emissions measured in the workplace. Furthermore, it is not classified as a combustible hazardous substance, a fact that makes transportation and storage of the solutions prepared (with this methyl ester) very easy. In addition, during casting, almost none of the undesirable gaseous breakdown products form since the numerous double bonds (rapeseed oil contains predominantly mono- und poly-unsaturated fatty acids) react to form solid compounds which do not evolve gas. When rapeseed oil methyl esters are used as the solvent, the maximum permissible exposure limits are not even approached. Furthermore, rapeseed oil methyl ester has an excellent release effect and thus facilitates the removal of cores and molds, which obviates the use of additional release agents.
The same applies to the other fatty acid methyl esters and fatty acid methyl ester mixtures. Due to its easy processibility, the methyl ester of soybean oil deserves special mention. Particularly satisfactory results were obtained with the methyl ester of linseed oilxe2x80x94in some cases even better than with rapeseed methyl ester. Castor oil methyl ester is a particularly suitable solvent for phenol resin but, due to its content of OH groups, It is less satisfactory for polyisocyanates and, on the other hand, has the advantage that, owing to these OH groups, it is incorporated in the polyurethane. Other methyl esters are listed in Table I.